Voicebanks are tools. You wouldn't use a hammer to fix a loose bolt right? A soft voice or a childish voice can be great assets if you choose the right songs. And powerful voicebanks aren't always as versatile as they may seem to be.

If you're referring to appends, here are some tips!
1. pick a theme based on the UTAU's name (ex. UTAU's name is Hana = flower names?)
2. pick a theme based on something you like (ex. I chose space names like Stardust/Comet)
3. pick a theme based on the UTAU's design (ex. Ayaka Sanshoku's theme is primary colors = VBs are Red/Blue/Yellow)
If you're looking for a name for the UTAU itself, this might still work ^^